I have just done this on a engine that was removed out of the car, Yours may not be as stuck as this one. It was a heavily rusted LB block ,rust on the upper bore .The previous owner had Soaked the bores in oil wD40 ?? .I removed all the Rod and main caps and crank, Then drove 6 pistons down then two up these were the center ones that cannot go down because they hit the webbing of the center main and they were near the top and had not much rust above then ,It worked out well the previous owner could not see how the pistons would come out ,I would use something like vinegar to soak them in next time ,by driving them down you are not trying to force them past heavy rust . Also I drove a screw driver under the lifters and cam the , the lifters all stayed up so the cam could be turned. You could soak yours in a light oil .Or wth heavy rust soak it in vinigar for 2 weeks then I would remove the heads then polish the bores with a drill and wire brush, then asses what needs doing next .You can nock the piston out with a 2x3 ,Ted